Advanced composites are high strength, high modulus materials which are finding increasing use as structural components in aircraft, automotive, and sporting goods applications. Typically they comprise structural fibers such as carbon fibers in the form of woven cloth or continuous filaments embedded in a thermosetting resin matrix.
Composite properties depend on both the matrix resin and the reinforcement. In unidirectional carbon fiber composites, important mechanical properties include longitudinal tensile strength and modulus, transverse tensile strength and modulus, and longitudinal compressive strength. The matrix affects all of these properties, but has the greatest effect on compressive strength and transverse tensile properties. High composite compressive strengths and transverse tensile moduli require that the matrix have a high modulus.
State-of-the-art epoxy matrix resin systems in advanced composites are typically based on N,N,N',N'-tetraglycidyl 4,4'-diaminodiphenyl methane and 4,4'-diaminodiphenyl sulfone. These resins produce unreinforced castings which have tensile strengths of about 8,000 psi and tensile moduli of 500,000 to 550,000 psi. Unidirectional composites containing 60 volume fraction carbon fiber made with these matrix resins typically have transverse tensile strengths of 5,000 to 7,000 psi and transverse tensile moduli of 1.0 to 1.4 million psi. Higher transverse properties are particularly desirable for applications such as pressure vessels. Improved compressive properties are desirable for structures subjected to high compressive loads such as sucker rods for oil wells.
Epoxy resin systems affording higher matrix properties than state-of-the-art formulations are known. For example, U.S. Pat. No. 3,398,102 discloses tacky, curable polymers formed by reacting bis(2,3-epoxycyclopentyl)ether with aliphatic polyols. Castings made by curing these compositions with aromatic amines have some of the highest tensile strengths (16 to 18,000 psi) and tensile moduli (700 to 850,000 psi) of any thermosetting material. However, these castings typically have relatively low heat deflection temperatures and absorb large amounts of moisture. In addition, they cure relatively slowly, limiting their utility in certain composite fabrication processes such as filament winding. Thus, there is a need for matrix resins which afford high tensile strengths and moduli in combination with improved heat deflection temperatures, faster cure rates, and a reduced tendency to absorb moisture.
It has now been found that compositions containing a select class of cycloaliphatic epoxides in combination with reaction products of aromatic active hydrogen containing compounds with these same cycloaliphatic epoxides afford unreinforced castings with higher heat deflection temperatures, faster cure rates and lower water uptake than similar compositions containing epoxy adducts made from aliphatic polyols.
Further, it has been found that a particular combination of a cycloaliphatic epoxy resin with the reaction product of such cycloaliphatic epoxy resin and an aromatic active hydrogen containing compound can be used to produce unreinforced castings which have higher tensile modulii than a casting produced using the cycloaliphatic epoxide alone.